Physical activity (PA) is essential for cardiovascular health, cognitive health, and independence in aging. Both aging and osteoarthritis (OA) are independently associated with insufficient PA but it is not known how these two factors may work together to adversely impact PA. As the number of older adults with hip OA surges, there is an urgent need to identify modifiable factors that decrease PA so that targeted PA-promoting interventions can be developed for this group. Our long-term goal is to develop interventions to increase PA in older adults with hip OA. A hallmark of hip OA is hip abductor muscle atrophy, weakness, and abnormal muscle firing patterns. The objective of this study is to determine how PA limitation may be influenced by dynamic hip abductor impairment during walking. The hip abductors are responsible for maintaining a level pelvis during the single limb stance phase of gait. When this is not possible, individuals typically compensate by exaggerating the excursion of the trunk in the frontal plane. While effective, these gait patterns increase the energy cost of walking. The central hypothesis of this study is that hip abductor impairment is associated with decreased PA because compensatory increased trunk motion increases the energy cost of walking, which results in greater fatigability. This hypothesis builds on our preliminary data, including recent work linking hip abductor function to gait limitations and the energetic model of activity limitation in older adults. According to this model, reduced PA in older adults may be a result of the decrease in total available energy that accompanies aging, and people who use an excessive amount of their available energy during ordinary activities, such as walking, are less likely to have sufficient energy reserve to engage in recreational or lifestyle PA. This model has already led to successful interventions for older adults with nonspecific mobility impairments. Our key innovation is to use the model to identify modifiable OA-specific mobility impairments that reduce PA. To achieve our objective, we will evaluate 90 women age ? 60 with symptomatic doctor-diagnosed hip OA using gait analysis, measuring oxygen consumption, and assess perceived and performance fatigability, along with background variables known to affect PA, such as pain and BMI. We will address our hypothesis through two aims: 1) Determine the extent to which impaired hip abductor function, or compensations for impaired hip abductor function, increase perceived and performance-based fatigability measures, and 2) Determine the extent to which impaired hip abductor function, or compensations for impaired hip abductor function, increase the energy cost of walking and activity limitation in women with hip OA. This study will identify the elements of a new intervention that improves function and promotes PA. This study will provide a new mechanistic understanding of the interactions among aging-associated functional impairment, OA-associated functional impairment, fatigue, and PA, and establish a new framework for understanding PA limitation in older adults with and without musculoskeletal impairments.